1. Field of the Invention
This invention relates to a multilayer sheet material suitable for the packaging of foodstuffs and more particularly to a polymeric multilayer sheet material particularly adapted for use in making containers for food products to be cooked or reheated in microwave ovens.
2. The Prior Art
The widespread popularity of microwave ovens has initiated interest in a plastic container which can be used for the packaging of foods and which can withstand the temperature demands of cooking and reheating the food product in microwave ovens. Microwave ovens are popular with consumers because of the speed at which such ovens can cook or reheat food. Containers used in microwave ovens are heated only by the heat conducted from the food being cooked. Although the temperature demands of microwave cooking are less than that of conventional ovens, the temperatures that are generated from the food product still deleteriously affect the dimensional stability of containers formed from single or multilayer plastic sheets thermoformed from polyolefinic materials such as propylene polymers and copolymers.
In addition to plastic containers made solely from polyolefinic materials as just described, the art also has under consideration containers for the packaging of oxygen-sensitive foods intended for storage under non-refrigerated conditions which can be subject to microwave conditions. These containers are formed from multilayer sheets constructed of an inner barrier layer, juxtaposed adhesive layers on both sides of the barrier layer and outer layers of a polyolefin. For example, British Pat. No. 1,379,106 discloses a five-layer sheet comprised of an inner gas barrier layer formed of a copolymer of vinylidene chloride and vinyl chloride, an ethylene/vinyl acetate copolymer adhesive layer on either side of the barrier layer and an outer layer of a polyolefin such as polyethylene, polypropylene or ethylene/propylene copolymer. The multilayer sheets are disclosed as being ideally suited for making containers of a variety of forms for products that are sensitive to oxygen.
The multilayer sheets of the prior art are conventionally made in a single process step by coextruding two or more polyolefinic layers and intermediate layers comprised of an adhesive tie layer material into a continuous sheet to form a unitary structure. In the case of multilayer gas barrier sheets, an inner layer of a gas barrier material is coextruded with the polyolefinic and adhesive layers.
A multilayer sheet structure having gas barrier properties which has been found particularly suitable for the manufacture of plastic containers used for the packaging of oxygen sensitive food products is constructed of an inner gas barrier layer such as ethylene vinyl alcohol (EVOH) or vinylidene chloride polymer (SARAN), first and second adhesive tie layers applied to each side of the barrier layer, a first outer layer of low or high density polyethylene, bonded to the first adhesive layer and a propylene polymer or styrene polymer layer bonded to the second adhesive layer.
Containers thermoformed from such multilayer gas barrier sheet are utilized in the packaging of oxygen-sensitive food products which are intended for storage under non-refrigerated conditions. In such application, the containers are formed using a sterile multilayer gas barrier film, and while in a still heated state from the forming station are filled with sterile food product in a filling station inside a sterile enclosure. The filled containers are then sealed without it being possible for their contents to be contaminated by a non-sterile atmosphere with a lid sheet whereof at least the side of the lid facing the inside of the containers is sterile. A specific method which may be utilized for sterile packaging of the type generally described above is disclosed in U.S. Pat. No. 4,287,802.
Multilayer sheets of the non-barrier and gas barrier type can be converted to containers using conventional thermoplastic forming techniques such as thermoforming. In thermoforming, the multilayer sheet material is thermoformed by feeding the sheet from roll stock past a heating station where the sheet is heated, usually by passage through an oven or overhead radiant heaters or a combination of both and the heated sheet passes directly to a thermoforming machine which forms the container. Any suitable thermoforming technique such as vacuum forming, pressure forming, plug assist or mechanical forming or any combination of such techniques can be used in the forming of the containers but in all cases the sheet material is preheated generally to temperature of about 375.degree. F. to about 425.degree. F. to effect a degree of softening of the material so that it can be easily thermoformed into containers having uniform sidewalls of the desired thickness.
One drawback to the plastic containers formed from multilayer sheet of the types above described is that the containers are not microwaveable, i.e. the containers when filled with food product and exposed to heating in a microwave oven exhibit poor dimensional stability and often undergo distortion and shrinkage.